The purpose of this study is to develop defective interfering HIV-1 particles that are able to target HIV-1 expressing cells and to interfere with HIV-1 replication in these cells. These particles would use wild type virus as a source for all structural proteins to generate new defective viruses . These in turn will then repeat this cycle in other HIV-1 infected cells. The overall goal is to reduce the HIV-1 load in the patient over a long period of time and thereby delay the onset of AIDS. Many complex elements of this antiviral strategy must be studied in detail at the molecular level. In the past, we have developed defective interfering HIV-1 proviral DNAs that interfere quite effectively with wild type virus in tissue culture. During the past year, we have constructed several new candidate defective interfering proviral DNAs. We are in the process of coexpressing these DNAs with wild type HIV-1 DNA to demonstrate for the first time a transfer of the interfering genes through infection by defective interfering HIV-1 particles. For this purpose, a hygromycin resistance gene was inserted into the defective HIV-1 provirus under control of a TK promoter. Expression of this resistance gene will be independent of HIV-1 Tat protein. Transfer of the resistance and the interfering genes through infection, however, will be totally dependent on the structural proteins provided by wild type HIV-1. For the future targeting of these defective interfering HIV-1 particles through pseudotype virus formation, we have synthesized several new candidate chimeric CD4 proteins. These proteins will be evaluated with respect to their efficiency of expression, transport to the cell surface, insertion into HIV-1 particles and subsequent binding of these particles to HIV-1 Env expressing cells, followed by infection of these cells and defective interfering HIV-1 proviral insertion. These studies will be most important to understand pseudotype virus formation and the development of targeted viral vectors for postmitotic cells of the CNS.